The UMTS terrestrial radio access (UTRA)—time division duplex (TDD) system is based on a combination of code division multiple access (CDMA) and hybrid time division multiple access (TDMA) which is known in the art as code-time division multiple access (CTDMA). (UMTS is an acronym for universal mobile telecommunication system also known by persons skilled in the art.)
In time division multiple access (TDMA), the time axis is divided into a plurality of time slots and users are permitted to transmit in certain ones of those time slots. Code division multiple access (CDMA), on the other hand, utilises direct sequence spread spectrum (DSSS) where a data is multiplied by a spreading factor (Q) prior to transmission in order to encode the data.
Current mobile telecommunications systems comprise a plurality of telecommunication cells each having at least one base station and at least one mobile terminal. Both the base station and the mobile terminal are capable of functioning as a transmitter and a receiver of radio frequency (RF) signals.
In a CTDMA system a number of users are permitted to transmit data sets during the same time slot. Each data set in a given time slot is separated by code division.
In order to receive transmissions in each time slot, the receiver must be able to estimate the characteristics of the transmission channel between the transmitter and receiver. In mobile telecommunications applications multipath distortion can arise and training sequences or some known data content is needed to facilitate channel estimation in the receiver. The information needed to estimate the transmission channel's characteristics is contained in the channel's impulse response, a term well known in the art. The training sequence may be referred to as a Training code, Midamble code, or Pilot code, all terms well known in the art. For the purposes of the following discussion of the prior art and disclosure of the present invention, the term Training code will be used when referring to the training sequence as defined above.
The Training code is required on both the uplink from mobile terminal to base station and the downlink from the base station to the mobile terminal in order that each transmission path to and from each user can be estimated.
A separate and distinct code from the Training code is the Channelisation code. The Channelisation codes are orthogonal codes taken from the set of Walsh codes and is a term well known in the art. The Channelisation code contains an index number and a spreading factor, both of which serve to encode the data prior to transmission. Thus the Channelisation code is also required during both uplink and downlink in order to interpret a user's data.
To enable multiple spreading factors to be applied to different user's data within the same time slot the Channelisation codes are assigned in a particular way. The preferred method of assigning Channelisation codes, which is well known in the art, is the Orthogonal Variable Spreading Factor (OVSF) code relationship and is shown in the tree-link structure in FIG. 1.
Currently during the downlink, a single Training code is transmitted along with all the data sets of the users in a given time slot. The receiver detects all these data sets along with the single Training code. This is known in the art as Joint Detection and it increases the probability of detecting the data set of interest to a particular mobile terminal user. Having received all the data sets the Receiver must know all the active Channelisation codes in order to insure it is able to interpret the data set of interest.
A problem with the current method is that if the receiver is configured to detect 8 Channelisation codes but only 2 data sets are active, the performance of system will be decreased. Similarly if the receiver is configured to detect 2 Channelisation codes but 6 data sets are active, the performance of the system will be decreased.
Currently, on the uplink of a dedicated channel the base station will know both the Channelisation code and the Training code as it assigns them. The base station communicates these to the mobile terminal in the prior downlink transmission.
However, on the uplink of channels that are not dedicated, for example when the mobile terminal is first switched on, there is often no signalling mechanism to assign Channelisation codes and the mobile terminal will pick at random a Channelisation code to transmit with. Under these circumstances a fixed relationship between the training and Channelisation code must be adopted.